A conventional key sheet was a component (subassembly) constituting a keyboard portion of handy mobile equipment such as a portable phone, a personal digital assistant (PDA) or the like, and such key sheet was configured so that a plurality of key tops which are push buttons indicating an alphabetical and/or numerical key or a functional key were arrayed and adhered onto the top surface of a sheet-shaped member referred to as a keypad. A material for the above-mentioned keypad was a soft sheet having rubber elasticity such as a silicone rubber or a thermoplastic elastomer or the like.
On the undersurface of the keypad thus configured, small projections referred to as pressing elements are attached to the area corresponding to each of the above-mentioned key tops, and furthermore the pressing elements are closely disposed to a printed circuit board formed independently which has a plurality of normal open contacts closed by pressing the push button, and according to those, a push button switch is formed.
Since the key sheet is thus configured in the manner that a plurality of components are piled up in layers, the thickness of each component is accumulated, which causes substantial total thickness of the key sheet. Therefore, it is required that whole of the handy mobile equipment should be thinned more so as to enhance the further portability thereof.
For example, the Patent References 1 and 2 set forth a production method of a key sheet using a method so called the mold printing where the key sheets is formed by transferring and pressure-bonding a resin filled in a plurality of convex portions provided on the surface of a rotating cylinder to a substrate sheet.
[Patent Reference 1]
Japanese Laid-Open Patent [Kokai] Publication No. H-11-156871
[Patent Reference 2]
Japanese Laid-Open Patent [Kokai] Publication No. 2003-127225
However, since a key sheet formed by the production methods set forth in the Patent References 1 and 2 has a configuration by means of a conventional layered structure formed by superposing key tops or the like as each individual part (component) on a keypad having rubber elasticity such as a silicone rubber, a thermoplastic elastomer or the like, there is a structural limitation for further thinner configuration. That is, there is a limitation for further thinner configuration of the keypad having rubber elasticity such as a silicone rubber, a thermoplastic elastomer or the like in the view of ensuring a click feeling and a strength when pressing a normal open contact. Therefore, it is not possible to thin a keypad to 0.2 mm or less, even if the pressing element portion which is the thickest portion is excluded.
Further, a method of integrally forming a projection such as a pressing element or a key top on a film made of a thin resin using the above-mentioned mold printing can be considered. In this case, if the key top is formed by means of the mold printing, the pressing element is necessary on the opposed undersurface side of the film. However, it is not possible, in the mold printing, to form projections on both surfaces of a single film. Therefore, when forming the key tops and the pressing elements together by means of the mold printing, it is necessary, for example, to adhere together a film having the pressing elements provided on its one side by means of the mold printing and a film having the key tops provided on its one side by the mold printing.
In addition, a key top formed by means of the mold printing is not good in its operational feeling, since the key top is, in many cases, made of relatively soft materials such as PVC, acrylic UV-cured resins or the like.
Further, a method of forming key tops on one side of the sheet by means of the mold printing and forming pressing elements only by dropping a resin onto the back surface of the film using a dispenser or the like can also be considered. However, it is quite difficult to control precisely an amount of the resin to be dropped, since the pressing elements become very small only by the simple dropping method. That is, this method involves a problem in which the size of a pressing element is affected by an amount of the resin to be dropped.